1. Technical Field
The present invention relates to a spectrum analysis system and a spectrum analysis method. In particular, the present invention relates to a spectrum analysis system and a spectrum analysis method for measuring a signal component at each frequency of an input signal.
2. Related Art
A spectrum analyzer is a known apparatus for analyzing frequency of a signal, as shown in, for example, Japanese Patent Application Publication No. 2001-272425. This spectrum analyzer generates an IF signal by multiplying an input signal to be measured and a local signal having a frequency that sequentially changes, and passes a prescribed band component of the IF signal through a band pass filter. The spectrum analyzer then AD converts the signal component passed through the band pass filter, and displays the result in a frequency spectrum as a signal component at each frequency determined according to the frequency of the local signal. The frequency resolution of this spectrum analyzer is determined by the pass-band of the band pass filter immediately before the AD converter.
In order to realize a narrower resolution bandwidth, that is, in order to display the frequency component of the input signal with a resolution bandwidth narrower than the pass-band of the band pass filter immediately before the AD converter, the spectrum analyzer uses a digital filter to filter the digital signal after AD conversion, as shown in, for example, Japanese Patent Application Publication No. 9-257843, or performs an FFT on the AD converted digital signal.
When using a digital filter to analyze the frequency with a narrower resolution bandwidth, however, the spectrum analyzer must change the local signal at frequency widths less than or equal to the resolution bandwidth, e.g. frequency widths less than or equal to half of a half-value width of the digital filter.
Furthermore, when analyzing the frequency of the signal over a wide frequency span, the spectrum analyzer samples high-order harmonics of the IF signal obtained by multiplying the local signal by the input signal, and performs data processing on these sampled signals. In this case, the frequency change of the sampled high-order harmonic signal of the IF signal is several orders greater than the frequency change of the local signal. For example, if a 6th order harmonic signal of the IF signal is sampled and processed, the frequency change of this 6th order harmonic signal is 6 times the frequency change of the local signal. In other words, the spectrum analyzer must change the frequency of the local signal at frequency widths that are less than or equal to 1/12 of the resolution bandwidth.
If the local signal changes within a range of 4 to 8 GHz and the 6th order harmonic signal is sampled and processed, the spectrum analyzer can analyze the frequency of the input signal over a wide frequency range of 48 GHz. If the resolution bandwidth is set to 1 kHz, however, the spectrum analyzer must change the frequency of the local signal at frequency widths of 1/12 kHz over the range from 4 to 8 GHz. In other words, the spectrum analyzer must be provided with a local signal oscillator that can change the local signal with a resolution less than or equal to 100 Hz over the range of 4 to 8 GHz. A local signal oscillator capable of such a wide range and high resolution has a large circuit size.
On the other hand, if an FFT is used to analyze the frequency with a narrower resolution bandwidth, the spectrum analyzer can achieve a high resolution bandwidth without changing the frequency of the local signal at small frequency units. However, the spectrum analyzer can output only the signal component within the pass-band of the band pass filter immediately before the AD converter with a single FFT. Accordingly, when analyzing the frequency of the signal over a wide frequency span, the spectrum analyzer must perform a plurality of FFTs at units equal to the pass-band of the band pass filter. For example, when using FFTs to analyze the frequency of a 40 GHz span from 5 to 45 GHz, if the pass-band of the band pass filter is 10 MHz, the spectrum analyzer must perform the FFT 4000 times (40 GHz/10 MHz). Therefore, the spectrum analyzer that uses FFTs to analyze the frequency with a narrower resolution bandwidth requires an extremely long time to analyze the frequency over a wide frequency span.